I do not think that a homemade smart cover would work. In order for the magnets to work with the iPad there is a certain pattern of small rectangular magnets that each have a certain polarity to tell the iPad when the cover is activated and to keep it shut. These magnets are circular and will not work with the iPad. In iFixit's teardown of a smart cover they discovered that there are lots of little magnets. Look at step 2:
I am not trying to say it is impossible but you would have to get very specific magnets and in the end I think it is just better to buy a smart cover.
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